1. Field of the Invention
The present invention relates generally to screen printing techniques, and more specifically, to a metal mask plate used for forming a film of a desired pattern on a surface of a printed board or a semiconductor substrate by screen printing.
2. Description of the Related Art
More and more chip parts are employed as electronic parts mounted, for example, on printed boards having interconnection patterns formed thereon, for the purpose of achieving high performance by increasing integration density in installation, reducing weight and size, and removing lead wires. The chip parts themselves tend to be reduced in size.
Generally, prior to installation of chip parts onto a printed board, solder paste (cream solder formed by kneading fine particles of solder, binder, solution etc. into paste) is applied and stuck to desired parts of a surface of a printed board by means of screen printing or other methods.
Screen printing will be described briefly hereinafter. Screen printing, as illustrated in FIG. 1, uses a printing plate 2 attached to a screen frame 1 under prescribed tension. Ink 3 is applied onto printing plate 2. A material to be printed 5 is placed under printing plate 2 with a gap G therebetween. A squeegee 4 is pressed against the top surface of printing plates 2 and slides thereon, and printing plate 2 is pressed into contact with the material 5. While the printing surface is in contact with the surface of material 5, squeegee 4 is moved in the direction of arrow. Thus, the ink 3 is squeezed out onto the bottom surface of printing plate 2 through openings formed in printing plate 2 to be transferred onto the material 5, and a desired pattern is formed on the surface of the material 5.
A screen printing plate used for applying solder paste to desired parts of a surface of a printed board is generally produced as follows. A metal thin plate is etched utilizing a mask according to a desired pattern. A metal mask is obtained which has a printing pattern corresponding to the above-described desired parts formed of openings penetrating in the thicknesswise direction of the thin metal plate, and the non-opening part. The metal mask will be referred to as a patterned member. As illustrated in FIG. 2, metal mask 6 is joined to a combination member 7 formed of an elastic material such as synthetic fiber mesh, and the circumference of combination member 7 is fixed to a screen frame 8 to produce a screen printing plate. A solder paste film applied by utilizing such a screen printing plate is formed according to a commonly known screen printing technique as described above.
More specifically, application of solder paste to desired parts of a surface of a printed board by the above-described screen printing technique will be described. As illustrated in FIG. 3, solder paste 11 is placed on a metal mask plate 9 having openings 10 formed according to a pattern. Metal mask plate 9 is lowered by a squeegee 12 into contact with the surface of a printed board 13 which is a material to be printed. While metal mask plate 9 is in contact with printed board 13, squeegee 12 is made to slide across mask plate 9 in the direction indicated by the arrow. With the traveling of squeegee 12, the solder paste 11 is squeezed out to the bottom surface of metal mask 9 through the openings 10 of metal mask plate 9. The solder paste is transferred onto the surface of printed board 13 and a solder film is formed according to a desired pattern.
However, when the solder paste 11 is squeezed out to the bottom or rear surface side of metal mask plate 9 through opening 10, part of the solder paste moves to the rear surface side (the surface in contact with the surface to be printed) from the opening 10 of metal mask plate 9. As illustrated in FIG. 4, paste oozes from the edge of opening 10. Therefore, when the solder paste 11 is transferred onto the surface of printed board 13 through the openings 10 of metal mask plate 9, unnecessary paste 16 disadvantageously sticks to the circumference edge of the transferred paste 15 of the printed board surface.
This phenomenon is generally encountered when screen printing is performed utilizing such a metal mask plate 9. This phenomenon is one of the things which have to be especially taken care of in screen printing utilizing a metal mask plate. This restricts the number of substrates which can be screen-printed successively without washing the rear surface side of the metal mask plate. The metal mask plate must be washed every prescribed number of printings, and therefore work efficiency is poor.
One possible solution to this disadvantage is to change the fluidity of paste in order to restrain the solder paste from moving into the rear surface side of the metal mask plate to the utmost. For example, the use of highly viscous solder paste is considered. However, this makes it difficult for the solder paste to pass through the opening of the metal mask plate. It is particularly difficult to print fine patterns according to this method.
Furthermore, the amount of paste oozed from the edge of the opening of the metal mask plate is not constant. It is therefore not easy to set conditions for printing.